EXTERNAL AND MIDDLE EAR. 93 



of semi-fluid ground substance and branching corpuscles, except along one border, 

 where they are in continuity. But in the cochlea the gelatinous tissue is above and 

 below the epithelial tube, the place of the modiolus being occupied by embryonic 

 tissue which is not gelatinous, and is connected with that lining the capsule by 

 similar non-gelatinous tissue separating the turns of the cochlea from one another, 

 and also running in the position of the future spiral lamina. 



The bone, which is formed by ossification of the cartilaginous capsule, is of a spongy 

 nature, but it becomes coated internally by layers of compact bone deposited by the periosteal 

 lining. The modiolus and septa of the cochlea, as well as the osseous spiral lamina, are 

 formed wholly in connective tissue without any pref ormation in cartilage. 



The perilymphatic spaces throughout the whole labyrinth are produced by a 

 gradual vacuolation and disappearance of the gelatinous tissue which surrounds the 

 membranous labyrinth. In the cochlea this conversion into perilymph begins in the 

 proximal turn of the spiral and extends hence towards the distal end. It is only 

 with the development of these perilymph-spaces (scalae) that the cochlear tube, 

 which was previously oval in section, acquires the characteristic triangular section 

 which we see in the fully-formed organ. 



The auditory nerve is large and early becomes separated into its two main divisions, vesti- 

 bular and cochlear. Each division has a large ganglion upon it (fig. 105), which extends 

 to the anterior wall of the epithelial vesicle, and as the ventral end of the vesicle elongates 

 and assumes the spiral disposition, the cochlear nerve and ganglion extend along with it and 

 take the same coiled or spiral form. 



The cells which form the wall of the epithelial tube become variously modified in different 

 parts of the labyrinth to produce the characteristic structures which there occur, viz. : the 

 hair-cells, the rods of Corti, the sustentacular cells of Deiters and the epithelium lining the 

 labyrinth. The membrana tectoria appears as a cuticular deposit over the columnar cells 

 which are becoming developed into the organ of Corti. 



ACCESSORY PABTS OF THE ORGAN OF HEARING. EXTERNAL AND MIDDLE EAR. 



While the epithelium of the internal ear is formed by an involution of cutaneous 

 epiblast in the manner which has just been explained, the middle ear with the 

 Eustachian tube, and the external auditory meatus with the pinna are formed from 

 the remains of the first visceral cleft, and from the parts of the mandibular and 

 byoidean arches which immediately bound the cleft. This cleft at an early period 

 forms an almost complete communication between the pharynx and the exterior, 1 but 

 the broad cleft becomes gradually converted into a flattened tube, and this is presently 

 found to be closed, both by the epiblast and hypoblast, which are from the first 

 in contact at the bottom of the cleft, and also by an ingrowth of mesoblast, the 

 rudiment of the membrana tympani being thus formed. There is at first no enlarge- 

 ment of the flattened tube to represent the tympanic cavity, and the ossicles are 

 developed not within, but altogether outside the tube, in a mass of gelatinous con- 

 nective tissue, which is continuous with that forming the embryonic membrana 

 tympani ; they are formed for the most part by ossification of parts of the carti- 

 laginous bars, which extend from the otic capsule into the mandibular and hyoidean 

 visceral arches (see Development of Skeleton). As the tympanic cavity becomes 

 formed by a gradual enlargement of the blind end of the closed hyomandibular cleft, 

 the gelatinous tissue retires before it, and as this tissue disappears, the ossicles and 

 the chorda tympani which were previously entirely enveloped by it, are left projecting 

 into the tympanic cavity, covered only by thin mucous membrane. The process of 

 formation of that cavity is not, in fact, completed until after birth, when air becomes 

 admitted into it through the Eustachian tube. 



1 Vide footnote on p. 102. 



